RF communication systems, such as trunked radio communication systems, are well understood in the art. Such systems typically provide a plurality of communication resources (such as frequencies, frequency pairs, or TDM time slots) that can be allocated on an as needed basis amongst a group of member communication units by a resource controller. Generally, such as system will provide resources that support both user voice and data messages as well as control signalling to support this allocation of the resources in an orderly and efficient manner.
In such a system, a private cell between a source communication unit and a target communication unit can be established when the source communication unit initiates a talk/request message in the appropriate control signalling format. This message will typically include the ID of the source unit and the ID of the target unit. The resource controller will then cause this same information to be retransmitted along with a resource grant identification. The source and target units receive this message and begin communicating on the identified resource.
Location determining mechanisms, such as Loran C based units, are also well understood in the art. Such units function to receive Loran C signals from three separate locations, and by an appropriate calculation, determine the relative geographic location of the unit. This location information can then be transmitted through a radio communication system for use at other locations, such as a dispatcher site. Typically, such location information is either provided independently by the mobile unit itself (for instance, everytime the user sends an ordinary voice message, or at regular time intervals), or upon request by the remote site. The latter process is known as polling.
A problem exists when integrating location derivation cable communication units into existing trunked radio systems. In particular, the existing trunking system protocol does not provide for a specific polling instruction that will cause only location derivation capable communication units to respond with only the requested location information. Although such systems could be redesigned to accommodate such an instruction, in general, such infrastructure alterations are costly and occasionally trouble prone.
A need exists for a way of allowing only location information to be polled in a trunked communication system without disrupting normal voice communication protocols and without requiring modification to the infrastructure of essential protocol itself.